An empirical solvus for CO2-H2O-2.6 wt% salt

Gwchimicaa Cosmochimica Acra Vol 45. pp 225 10 228 0 PergamonPressLtd 1981 Printedin Great Britain

An empirical solvus for C02-H&B-2.6

wt% salt

EVA MARIE HENDELand LINCOLN S. HOLLIS’IZR

Department of Geological and Geophysical Sciences, Princeton University, Princeton, NJ 08544, U.S.A. (Received 13 March 1980; accepted

in revisedform

8

October

1980)

Abstmct-The solvus in the system C02-H20-2.6 wt% NaCl-equivalent was determined by measuring temperature of homogenization in fluid inclusions which contained variable COI./H,O but the same amount of salt dissolved in the aqueous phase at room temperature. The critical point of the solvus is at 340 + S“C, at pressures between 1 and 2 kbar; this is about 65’C higher than for the pure C02-HZ0 system. The solvus is assymetrical, with a steeper H,O-rich limb and with the critical point at mole fraction of water between-O.65 and 0.8. . INTRODUCIION

UN~L RECENTLY, the composition of the fluid phase during metamorphism could only be inferred from thermodynamic models based on experimental data pertaining to the mineral assemblages. Studies such as those by TOURET(1974), RICH (1975), HOLLISF and BURRUSS (1976), BURRUSS (1977). CRAWFORD et al. (1979) and HOLLKTER er al. (1979) strongly suggest

that fluid phases present during metamorphism are preserved as fluid inclusions in some metamorphic rocks. Microthermometry provides a’means to determine the composition and density of fluids in the fluid inclusions (POTY et al., 1976). Interpretation of these data relies upon several assumptions, including the assumption that the fluid phase was homogeneous at the time of entrapment. In the pure CO*-HZ0 system, which has a solvus with a critical temperature at about 275”C, entrapment of a homogeneous fluid implies entrapment at temperatures outside the solvus for the appropriate fluid composition. At lower temperatures, within the solvus, an HzO-rich fluid and a CO1-rich fluid occur as separate phases which can be entrapped as separate, homogeneous fluids with the same temperature of homogenization, the temperature of entrapment; but one will homogenize through disappearance of the CO*-rich phase and the other through disappearance of the H1O-rich phase. If variable mixtures of the two fluids are entrapped in inclusions, homogenization temperatures would also vary and would be higher than the temperature of entrapment. The size and shape of the solvus in the pure C02-Hz0 system is known as a function of pressure up to 3.5 kbar (TODHEIDE and FRANCK, 1963). The solvus crest is elevated by more than 75°C at 1 kbar with 6 wtsd NaCl in the aqueous solution at room temperature (TAKENOUCHI and KENNEDY, 1965). Measurement of homogenization temperatures in fluid inclusions also has demonstrated that the solvus is expanded if other components are present. For example, HOLLISTERand BURRLW (1976) found that for impure CO2 + Hz0 systems, the solvus crest was 225

at least 80°C higher than for the pure system. However, data are lacking for the effect of known amounts of other components on the C02-HZ0 solvus. The present study presents empirical data defining the solvus for the system COZ-H+2.6 wt% NaCl (equivalent) in the aqueous phase at room temperature, based on determinations of composition and density of naturally-occurring fluids with variable COZ/I-120 ratio. ANALYTICAL

PROCEDURE

Microthermometric measurements were made on fluid inclusions in approximately 1 x 1 cm chips of doubly polished sections, 0.25 mm thick, using a Leitz microscope equipped with a Chaixmeca heating and cooling stage. Standard operating procedure is discussed by ROEDDER (1962) and TOURET (1977) and a detailed description of the equipment and modifications is presented by POTYer al. (1976). and BURRUS (1977). The stage is calibrated by observing phase changes of known compounds over the temperature range of interest. All observations were made with a 50 x U-stage objective, but through a 170 pm thick glass disk rather than a glass hemisphere, and with 16 x oculars and a 1.25 x lens between the objective and oculars; effective magnification is 640 x . The objective is cooled during heating runs by water circulating through a brass cooling coil wrapped around the objective. The data obtained for the present study are given in Table 1. A heating rate of 0.1 to O.Z”C/min was used for all observations of phase changes, and each measurement was repeated at least twice to determine precision of measurement. Precision varies with temperature and the fluid inclusion. The length of the vertical bars for the values plotted in Fig. 1 reflects this variation of precision. RESULTS

The inclusions in this study occur along three healed fractures, AS-S, BS-1, and BS-2, in concordant veinlets of quartz from a kyanite zone pelitic schist from Morse Basin, British Columbia (CRAWFORDet al., 1979). Formation of the fractures and entrapment of the observed fluids probably occurred late in the retrograde history of the schists. For the present study, details of the occurrence of these inclusions are not relevant to the conclusions; the existence of fluid inclusions, whose compositions and homogenization

226

EVA MARIE HENDEL and L. S. HOLLISIZR

Table 1. Fluid inclusion measurements

v

Inclusion number

TH

TM

AS-S-1 -2 -3 -4 -5 -6 -7 -a -9

H2° IO-15 10-15 10-15 75-80 75-80 10-15 75-80 10-15 10-15

co2 19.9 20.6 21.5 28.6 29.6 17.8 26.8 20.1 20.3

co2 -56.9 -56.8 -56.9 -56.8 -56.8 -56.8 -56.8 -56.9 -56.8

BS-1-l -2 -3 -4 -5 -6 -7 -9 -10

55-60 15-20 15-20 35-40 35-40 55-60 35-40 85-90 75-80

22.2 16.8 17.8 17.9 21.2 21.2 19.2 25.7 27.9

BS-2-11 -12 -13 -14 -15 -16 -18

75-80 30-35 30-35 75-80 30-35 30-35 75-80

23.7 12.8 14.0 24.2 17.6 19.6 25.0

v,

volume

40%;

TH,

of

final

melting

mole

(D),

fraction.

the C02-rich those

not

phase

of

phase,

labelled

(G),

cm’lmo le

H20 0.23-.35 0.26-.35 0.29-.3a 0.91-.92 0.91-.92 0.2-.29 0.9-.92 0.26-.34 0.26-.34

214(D) 274(G) 273(G) 270(L) 263(D) 273(D) 271(L) 270(G) 274(G)

0.74 0.80 0.79 0.79 0.75 0.75 0.78 0.71 0.65

24.7-26 38.5-41.7 38.5-41.7 30.3-32.3 30.3-32.3 24.7-26 30.3-32.3 19.4-20 21.3-21.7

0.81-83 0.29-.39 0.32-.41 0.57-.65 0.65-.70 0.81-.85 0.59-.66 0.93-.95 0.90-.91

340 286(D) 276(D) 262(D) 272(D) 308(D) 328 264(D) 291(L)

0.73 0.83 0.83 0.73 0.80 0.77 0.72

21.1-22 30-32 30-32 46-47.5 29-31 29-31 45-48

0.91-.93 0.52-.60 0.52-.60 0.91-.93 O.Sl-.60 0.56-.60 0.91-.93

277(L) 322(D) 340 272(L) 316(D) 320(D) 292(L)

8.7 a.7 a.7 a.8 a.8 8.7 a.7 8.6 8.7

-56.8 -56.8 -56.7 -56.7 -56.8 -56.8 -56.a

8.1 a.7 8.8 a.7 a.7 8.7 8.7

(‘C);

close

of

homogenization

D, density inclusion to

the

to the

’

(‘C);

(pm/cc);

v,

critical

TM,

molar

decrepitated, H20-rich

TH final

43.5-47.6 43.5-47.6 43.5-47.6 21.3-22 21.3-22 43.5-47.6 21.3-22 43.5-47.6 43.5-47.6

-56.8 -56.8 -56.8 -56.8 -56.8 -56.8 -56.8 -56.8 -56.8

and CL),

too

CO2 0.77 0.76 0.75 0.60 0.59 0.7Y 0.70 0.77 0.77

and homogenized

were

v

a.7 a.7 a.7 8.6 8.6 a.7 8.6 a.7 8.6

temperature

% at

temperature

P

TM clathrate

and properties

phase,

temperature

volume;

homogenized

X, to

respectively;

to

identify

homogenization.

temperatures Can be determined, is all that is necessary. The inclwions are ovoid to equant in forin, 10 @I or less in longest dimension. Analyzed inclusions range in size from 6 to 10 pm. Along fracture AS-5 (Table 1) there is a bimodal distribution of volume % HZ0 (at 40°C) in CO*--HZ0 mixtures: lO-15% HZ0 and 75-SO’?; H20. This distribution and the small range of temperatures of homogenization (270-274°C) strongly suggest entrapment of two distinct fluids at the time &f crack healing, and that the entrapment occurred at about 272’C. Fracture BS-2 has a bimodal distribution 30-35”~ HZ0 and 75-80% H,O; however the two groups do not homogenize at the same temperature. The H@rich inclusions homogenize between 272 and 292°C (near the temperature of the inclusions in fracture AS-Q but one from the H20-poor groups homogenizes at 340°C. The others decrepitated at 316-32zOC, prior to homogenization, implying higher temperatures of homogenization. This crack therefore contains inclusions representing at least two stages of equilibration. It is possible that the inclusions intermediate in composition have survived a recracking

episode, and that the CO*-rich fluid, presumably in equilibrium with the H20-rich fluid, was not found. Fracture BS-1 has inclusions with variable percent H20. Most decrepitated before homogenization, but three did not. Two of these have intermediate compositions (356004 H20). The range of compositions suggests heterogeneous entrapment of immiscible fluids. Final melting temperatures of CO2 (T,. COI) below that of the liquid-solid-vapor invariant point ( -56.6’C) in the pure CO2 system suggest the presence of a small amount of a component, such as CH,,, which is dissolved in CO2 liquid and gas but which is not present in the solids (HOLLISTER and BURRUS. 1976). An observed range of melting temperature of CO2 of about OYC is consistent with this suggestion. According to SWANENBERG (1979), the maximum mole fraction of CH* in the CO&h phase, for inclusions with the CO,-homogenization and CO,-final melting temperatures listed in Table 1, is 0.03. In the system C02-H#-NaCI, the salinity of the aqueous phase may be determined from the final melting temperature of the CO2 clathrate (COz. 5.75 H20). In the presence of pure liquid H20.

An empirical solvus for CO*-H#-2.6

I

I

I

I

0

1

.-3

? .Y

1 .T

c .J

c .c)

wt% salt

.I_

227

.a

.8

$0

Fig. 1. Empirical solvus for COz-Hz0 with 2.6wt% NaCl equivalent in the aqueous phase at room temperature. The solvus for pure COr-Hz0 of TODHEIDE and FRANCK(1963) at 1 kbar is shown for comparison. The length of the horizontal and vertical bars gives the estimated errors of measuremmt. The starred inclusions deerepitated; the homogenization temperature for these inclusions would have been at a temperature greater than that indicated.

liquid COZ, and gaseous COZ, the clathrate melts at

+ 10°C. COLLINS(1979) showed that measurement of temperature of ice melting in the presence of CO2 liquid and COz gas gives erroneous results for determining salinity. Addition of an electrolyte, such as NaCl, to the C02-Hz0 system depresses the univariant melting point of the clathrate, analogous to the depression of the Hz0 melting temperature with the addition of salt. The following equation, from Bozzo et al. (1973) is a least squares fit to CO1-clathrate melting point depression data as a function of NaCl content in the presence of CO2 gas and CO2 liquid: Wt% NaCl = 0.05286 (10 - t)(t + 29.361) where t = final clathrate melting temperature in “C, and 0 5 wt% NaCl I 16. From the data given in the table, clathrate melting at 8.7 f O.l”C implies 2.6 + 0.2 wt% NaCl equivalent in the aqueous phase of the inclusions. The actual species in the inclusions causing the lowering of temperature of melting of the clathrate are not known; however, they are reported as ‘NaCI equivalent’ in accord with the practice for reporting the lowering of melting of ice by unknown dissolved species in aqueous inclusions as ‘NaCl equivalent’. The relative volume percent of the CO2 phase in the total inclusion is estimated at +4O”C in the two phase region (CO1 vapor and CO1 liquid have hom-

ogenized, CO2 and Hz0 phases are immiscible). The pressure within the inclusion at 40°C is determined from the isochore which corresponds to the density of CO1 determined by the temperature of homogenization of the CO1 phase. This density and pressure, and the volume y0 H20, define the molar volume of. the fluid and the mole fraction Hz0 in the fluid. The largest component of error in determining mole fraction of Hz0 using this approach is in the visual estimation of volume percent Hz0 (R~EDDEI~, 1967). The volume % range given in Table 1, column 1 is based on comparing the size of the vapor bubble in the inclusions with the diagrams given by Roedder for estimating vapor bubble size for inclusions with known and regular shapes. The reported range in mole fraction does not include errors due to irregularities in the shape of the inclusions. Other sources of error in determining molar volume include neglecting the salt content (about l-2% error), neglecting the possible presence of a small amount of CH4 in the determination of CO*, and the uncertainty (about +O.lT) of the temperature of homogenization of the CO2 phases. The estimated errors in temperature of homogenization or decrepitation for each inclusion are given by the vertical bars in the figure. Homogenization to the HrO-rich phase is preceded by rapid motion of the final bubble ,of CO,; this motion is easy to see and hence the error is solely that for precision and accu-

EVA MARIEHENDELand L. S. HOLLISTER

228

racy of calibration of the stage. The homogenization temperature of those inclusions which homogenize to the COa-rich phase (CO2 bubble expands) are somewhat more difficult to determine. The uncertainties are largest for inclusions whose compositions are near those for the crest of the solvus. For these, the fading of the meniscus as the physical properties of the two phases approach each other increases the difficulty of seeing the exact homogenization temperature. The homogenization temperatures of all the inclusions were determined at least twice to check precision and to be sure the inclusions had not partially decrepitated during the first determination. Decrepitation of inclusions in the size range of those studied (6-10 pm diameter) is at pressures on the order of l-2 kbar (LEROY,1979). Because about 1 3 of the inclusions studied decrepitated, we estimate the pressures inside of the inclusions to have been approximately within this pressure range at homogenization. Because the temperature of the crest of the pure COz-HZ0 solvus varies through a temperature range of only 15’C between 1 and 3 kbar (TODHEIDE and FRANCK,1%3), we presume our solvus (Fig. 1) is applicable for at least the pressure range of l-2 kbar.

on the manuscript, and E. ROEDDERand J. TOURETfor their thoughtful reviews. Discussions with Bon B~RRu~~ throughout the course of this study are gratefully acknowledged. REFERENCES bzzo

A. T., CHEN

properties

The solvus of Fig. 1 is the empirical solvus for the CO*-Hz0 system with approximately 2.6 wtO, NaCl equivalent in the aqueous phase at room temperature. Its crest is at 340 + 5-C, between mole fraction of HZ0 of 0.65 and 0.8. This is 65 to 70°C higher than the solvus crest for the pure system between 1 and 2 kbar. Based on empirical results for the decrepitation of fluid inclusions, the solvus shown is valid for pressures between 1 and 2 kbar. Metamorphic fluids in the amphibolite facies of metamorphism, SOO-600°C can contain as much as 30-40q0 dissolved salts (CRAWFORDet al. 1979). Because only 2.6”, salt raises the crest by 65-70°C to 340°C. it is reasonable that fluids with higher salt contents may be immiscible not only in the greenschist facies of metamorphism ( < 500’C) but also into the amphibolite facies of metamorphism. However, more empirical or experimental data are needed to determine the degree of immiscibility in the system NaCI-H,O-C02. and to provide a basis for evaluation of the importance of fluid phase immiscibility to considerations of phase equilibria in metamorphic rocks. Ac~nawledyemenrs-This research was supported bv NSF grant EAR78-05887and formed part of an A.B. Thesis at Princeton by HENML(1979). We thank BARBARAMURCK and MARIA LUISACRAWFORDfor constructive comments

The and carbon di-

4th Int.

the

teds

and E. Delyannis), Vol. 3, 437-451. BURRU~~R. C. (1977) Analysis of fluid inclusions tic metamorphic from Bryant and Khtada C.: Thermodynamic and geologic interpretation of observed and molar volumes. Ph.D. Thesis. Princeton 156 COLLINS Gas hydrates and of freezing data for 74, 1435-1444. CRAWFORDhf.-L., KRAUS and HOLLISTER and fluid inclusion study Am. J. Sci. clusions PP. HOLLISTER

inA.B. Thesis. Princeton Univ., and BURRU~~ from Khtada Lake metamorphic Acta

HOLLISTER L. CONCLUSIONS

and BARDUHU

the hydrates

Sec.

163-175.

BURRUS~ C.. HENRYD. L.

HENDEL

102, 555-561. the evaluation when they decrepitate. Sot. Mineral. Crisrullogr. 102, 584-593. POTYB., LEROY and JACHIWOWICZ pour la mesure des sans le microscope: I’installation Sot. Mineral. Cristallogr. 99, RICH R. A. (1975) Harvard low temperature applications dual purpose freezing and 57, 1045-1061. ROEDDERE. (1%7) ore fluids. Ore Deposits ted. H. L. Barnes), pp. Mineral.

C’ristallogr.

and their application 68, 303-304.

TAKENOUCHI S.

KENNEDY The solubility NaCl solutions high temperaand pressures. Am. Sci. 263, 445454. T~DHEIDE and FRANK E. U. (1963) Das beit Kntsche Sys KoklendioxidWasser bis von 3500 bar. Z. Phys. 37, 388-401. T~URETJ. (1974) Facies granulite et fluids carboniques. In Geologic des Domuines Crisralins. pp. 267-287. Sot. Geol. Belgique. TOURETJ. (1977) The significance of fluid inclusions in metamorphic rocks. In Thermodynamics o/ j7uid inclusions in meramorphic rocks (ed. D. G. Fraser), pp. 203-227. Reidel.